Locating method and arrangement

ABSTRACT

The invention relates to a locating method and an arrangement for determining the location of a terminal in a radio system comprising a plurality of base stations and terminals. At least two base stations with known locations transmit a signal used in locating the terminal, and the signals are transmitted in beams with directional antenna patterns and the direction of the beams is changed as a function of time. The terminal determines the location of the base station and the transmission direction of the signal used in location determination by means of the beam transmitted by said at least two base stations. Since the terminal is located at the intersection of the beams of the base stations transmitting from known locations, the location of the terminal can easily be determined. The solution takes multipath propagation into account.

FIELD OF THE INVENTION

The invention relates to a locating method for determining the locationof a terminal in a radio system comprising a plurality of base stationsand terminals.

The invention further relates to a locating arrangement for determiningthe location of a terminal in a radio system comprising a plurality ofbase stations and terminals.

BACKGROUND OF THE INVENTION

A typical mobile telephone system covers a large geographical area andcomprises a plurality of coverage areas, i.e. cells, each usually servedby one base station. The cells vary greatly in size, usually inaccordance with the amount of the call traffic in the area. In areas ofhigh traffic density, the cell size is usually smaller than in areaswhere fewer calls are made. Irrespective of the size of a cell, it wouldoften be useful to know the geographical location or direction ofmovement of a subscriber terminal.

Several different methods have been developed for determining thegeographical location of terminals. The location information of aterminal can be utilized for a number of purposes. If the user desires,he or she can determine his or her location; the location of a terminalplacing an emergency call can be determined in order to be able to sendhelp; or call charging can be dependent on the location. The distance ofthe terminal from the base station communicating with it can easily bedetermined on the basis of propagation delay. Among the most widely usedmethods is a triangulation method in which the signal of the terminal ismeasured by means of three or more base stations, and the location ofthe terminal is computed on the basis of the propagation delay of thesignals. This solution is also known as the TA (Timing Advance) method.

In another known solution, the terminal receives information on theclocks and location coordinates of the base stations located in thevicinity of the terminal from the cellular system. Next, the terminalmeasures the time differences of the signals received from the basestations in question and determines its location on the basis of themeasured time differences and the information received from the system.

The problem of the known methods, such as the triangulation, is howeverthat the location of the terminal cannot be found out very accuratelyand the location information thus obtained cannot be utilized inapplications in which accurate geographical parameters are essential.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is thus to provide a method and anarrangement implementing the method so as to solve the above problemsand determine the location of a terminal accurately.

This is achieved by a method of the type disclosed in the introduction,the method being characterized in that it comprises the following methodsteps: at least two base stations with known locations transmit a signalused in locating the terminal; the signals are transmitted in beams withdirectional antenna patterns; the direction of the beam is changed as afunction of time; the terminal measures the signals transmitted by saidat least two base stations and used in location determination, on thebasis of which measurement the location of said base stations and thetransmission direction of the signal used in location determination aredetermined and the location of the terminal is determined on the basisof the location of the base stations and the transmission direction ofthe signal used in location determination.

The arrangement of the invention is characterized in that at least twobase stations with known locations comprise a transmitter fortransmitting a signal used in locating the terminal and having adirected beam, and the transmitter is arranged to change the directionof the beam as a function of time; the terminal comprises means in areceiver which are arranged to measure the signals which are arranged todetermine the location of the base station and the transmission angle ofthe signal used in location determination, and the radio systemcomprises means for determining the location of the terminal on thebasis of the measurement performed by the terminal.

Several advantages can be achieved by the method and arrangement of theinvention. The location of the terminal can be determined accuratelywith no need for great changes in the software of the base stations orterminals. The signalling load of the network part in the radio systemis light. The arrangement of the invention is more accurate than the TAor OTD (Observed Time Difference) methods, for example, and no activecall is necessary for determining the location. Furthermore, thearrangement of the invention operates equally well indoors and outdoorsand the terminal can also determine its location itself. In addition,the solution according to the invention can easily be combined withother known locating solutions.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described in closer detail in connection with thepreferred embodiments with reference to the accompanying drawings, inwhich

FIG. 1a shows an instantaneous situation in an embodiment of theinvention,

FIG. 1b shows an instantaneous situation in an embodiment of theinvention,

FIG. 2a shows a situation in which a signal used in locationdetermination is multipath-propagated,

FIG. 2b shows the timing of the signals received by a terminal and usedin location determination in the situation of FIG. 2a,

FIG. 3 is a block diagram of the terminal,

FIG. 4 is a block diagram of the terminal, and

FIG. 5 is a block diagram of a base station.

DETAILED DESCRIPTION OF THE INVENTION

The solution of the invention is particularly suited for a GSM radiosystem but it can also be applied to other radio systems.

Let us now examine the method of the invention at a general level. Thesolution is suited for a radio system comprising a plurality of basestations and terminals and in the method, at least two base stationswith known locations transmit a signal used in locating the terminal,which signals are transmitted in beams with directional antennapatterns. The direction of the beams is continuously or discontinuouslychanged as a function of time. A function which changes the directioncan be truly regular as in a constant rotating movement, periodic,pseudo-random or truly random. When the directioning is random, thedirection angle of the beam can thus be randomly chosen but duringtransmission, however, the angle is always known. The terminal measuresthe reception moment of the signal transmitted by at least two basestations and used in location determination. Since the location of thebase stations is known and since the transmission angles towards theterminal of the base stations are known, the location of the terminalcan be determined in the same manner as in triangulation. In thesolution of the invention, it is assumed that in view of the measurementthe terminal does not substantially move between the measurements of thelocating signals transmitted by different base stations.

The invention can now be examined by way of example without losing thebasic idea of the solution in accordance with the invention orrestricting the scope of the invention when the base stations rotate thesignal used in location determination steadily, in which case thedirection of the antenna pattern of the signals is changed as a functionof time by rotating the beam around the base station at the cell level.Since the angular velocity ω of the rotation is steady, the direction ofthe beam at an initiating rotational turn is ωT at moment T. In thesolution of the invention, at the beginning of at least one of therotational turns that follows the directioning of the beam can, however,also be changed by phase shift φ, in which case the direction of thebeam, i.e. angle θ, at moment T is θ=ωT+φ. Preferably, the beam does notthen point sequentially in the same directions during each round. Thisis useful particularly in discontinuous, i.e. discreet, rotation inwhich the direction angle θ without the addition of phase shift atmoment T is θ=nΔα, where n is an integer index increasing with time andΔα is a discreet change of the direction angle.

Let us now take a closer look at the solution of the invention by meansof FIGS. 1a to 4. FIGS. 1a and 1 b show the inventive solution in whichsignals 13 and 14 transmitted by base stations 10 and 11 and used inlocating a terminal 12 rotate around the terminals 10 and 11 in thedirection shown by the arrows. The previously known location of the basestations 10 and 11 is for instance determined on a cartesian axis ofcoordinates in such a manner that the base station 10 is situated atpoint (x₁, y₁) and the base station 11 at point (x₂, Y₂). At momentt=t₁, the beam 14 of the signal used in locating the base station 10points towards the terminal 12 situated somewhere within the area of thebeam 14. The direction angle of the locating signal 14 transmitted bythe base station 10 is then θ₁(t₁). FIG. 1b shows the situation atmoment t=t₂, when moment t₂ is different from t₁. The beam 14 of thelocating signal of the base station 11 then becomes pointed towards theterminal 12 at an angle θ₂(t₂). Further, the beam of the base station 10at moment t₁ is indicated by the broken line in FIG. 1b. Hence, on thebasis of the measurement, the terminal 12 is located at the intersectionof the beams 13 and 14 indicated by the vertical lines in FIG. 1b. Themain axes of the beams 13 and 14 are indicated by the dotted linesinside the beams.

In order to determine a more accurate location of the terminal theterminal 12 measures the intensity distribution or the like of the beamduring one or more turns and forms the direction of the main axis of thebeam. The main axis is then utilized in determining the location of theterminal 12 and the terminal 12 is thus situated at the intersection ofthe main axes which are indicated by the broken lines in FIG. 1b. If theintensity distribution of the beam is measured during one turn, thespeed of rotation must be rather low. The beam can thus rotate aroundthe base station 10 and 11 in for instance 36 seconds. When the width ofthe beam is 10° the terminal has approximately one second, in otherwords in the GSM system 8 idle TDMA time slots, i.e. 64 possiblelocating bursts, to receive locating information. If the terminal 12uses four different base stations in location determination, theterminal 12 is able to receive 16 locating bursts from each basestation. On the basis of these sixteen chronologically sequentialmeasurements the terminal 12 can form the intensity distribution of thebeams and determine the direction of the main axis.

The speed of rotation can also be slightly higher. In such a case, thewidth of the beam can for instance be 20° and the rotation can bediscreet, in which case the locating signal jumps ahead by 20° after thetransmission of each locating burst. Hence, a full turn takes 18 burstsand slightly over 10 ms. If the terminal 12 receives locating burstsonly in idle TDMA frames, the reception and transmission angles of alocating burst are independent from each other and thus the terminaldoes not always receive the locating burst from the same time slot.Receiving the locating burst in the same time slot would lead to asystematic error which would, on the average, correspond to half of theangle of opening of the beam. Also in this case the terminal collects asufficient number of statistical samples of the locating signalstransmitted by the base stations and forms the intensity distribution ofthe beam on the basis of the measurements and determines the main axisfrom the distribution.

In a fast rotation of the beam, the beam rotates a full circle in forinstance approximately 4.6 ms. The angle of opening of the beam is thenpreferably for instance 45°. As in the previous example, the beamrotates after the transmission of each burst by the angle of opening,i.e. in this case 45° Hence, the beam rotates around the base stationduring 8 bursts, i.e. one TDMA frame. At specific intervals, preferablyat intervals of 120 ms, the direction angle of the beam is changed byadding or subtracting for instance 2.5° in order to prevent the terminalfrom receiving the burst of the locating signal from the same time slotin each direction angle all the time. In accordance with this example,the rotating of the burst of the locating signal is repeated in asimilar manner in each 18 * 120 ms=2.16 s, during which time theterminal has preferably collected 18 samples of each locating signalswithin the 45° angle of opening. The intensity distribution can thus bepreferably determined with an accuracy of 2.5°.

In a preferred embodiment of the invention the base stations 10, 11transmit information on the base station and the direction angle θ withthe signals used in location determination. Since the location of eachbase station 10, 11 is known, the location of the terminal 12 can bedetermined by means of the direction angle of the locating signal of atleast two different base stations.

When the base stations 10, 11 and the terminals 12 are synchronized soas to make the reception moment of the signal and the transmission angleθ depend on each other, the base stations 10, 11 can only transmitinformation on themselves with the signals used in locationdetermination. The information on the base station can be a base stationidentification code by means of which the base station 10, 11 can belocated or the information can directly be the location information onthe base station.

The network part can be determined to comprise the other parts of theradio system except the terminals. When the terminal 12 then transfersits measurement information over a radio path to the network part, thelocation of the terminal 12 is determined in the network part. Inpractice, the location can be determined for instance at the basestation controller. As an alternative to determining the location in thenetwork part, the terminal can also determine its location itself.Particularly in the GSM radio system, the signal used in locationdetermination can be transmitted over a BCCH (BroadCastCHannel) channelor the BCCH channel can indicate which frequency and which time slotsare for the location, what the coordinates of the base stations are(identity/coordinate), etc. In such a case the network transmits thesignal used in locating the terminal at one frequency in one or moretime slots.

Multipath propagation, i.e. the fact that the signal propagates from thetransmitter to the receiver via several routes, presents a problem inthe radio locating systems. The inventive solution can mitigate theeffect of this problem, or even eliminate the problem.

The problem caused by multipath propagation can be mitigated forinstance in such a manner that the terminal 12 measures the signal usedin location determination during one full turn. In the case inaccordance with FIG. 2a, the terminal 12 for instance detects a signal(reflected) of a path 21, a signal (reflected) of a path 22, and asignal (direct, correct) of a path 23. Let us suppose that these are theonly signals that the terminal 12 detects during the time it takes thesignal used in location determination to rotate 360° around the basestation 10. The problem is to find out which of the detected signals(angles) 21, 22 and 23 is the correct one. The terminal 12 knows thatfor example in the GSM, the time difference between the signals is(during transmission) the multiple (n*0.577 ms) of a time slot (0.577ms). In general, the time difference between the signals can be anyknown or predetermined length of time. If the time between the receivedsignals (modulo time slot length, i.e. 0.577 ms in the GSM) is longer orshorter, the signals have traveled distances of different length. In thecase of FIG. 2a, which is illustrated in closer detail in FIG. 2b, thefirst signal 21 arrives at a moment which is set to moment 0. The nextsignal 22 arrives for instance at moment 3.1 (time slot length units),and the third signal 23 for instance at moment 7.9. The terminal 12 caninfer that if the signal 21 had come directly, all the other signals 22and 23 should be equally early or late. The signal 22 is late in regardto the signal 21 (the signal 22 should have arrived at moment 3.0 inorder to have propagated directly). The signal 23, on the other hand,should have arrived at moment 8.0. However, since the signal 23 isrelatively ahead in regard to the signal 21, it can be inferred that thesignal 23 is a signal that has propagated directly (or has come via theshortest reflected route). (In the GSM, the length of a time slot 0.577ms corresponds to 173 km. Hence, the relative arrival moment 7.9 of thesignal 23 cannot mean that the signal 23 should have arrived at moment7.0 and that it would be 0.9 time slots late since the signal 23 shouldthen have traveled 156 extra kilometers, which is impossible). Theterminal 12 can thus prevent problems caused by multipath propagationand signal reflection by measuring the relative time differences (modulotime slot length) between arriving locating signals during the period oftime it takes the locating beam to rotate 360°. The arrival moment ofthe first received signal is set to moment 0. If the next signals do notinclude signals that are early, the first signal is the most directsignal possible. If an early signal exists, the one with the longestadvance is the most direct signal.

FIG. 3 shows a block diagram of the terminal in accordance with theinvention. The terminal comprises an antenna 30, radio frequency parts31, a demodulator 32, means 33 for measuring the locating signal andlocating means 34. The radio frequency parts 31 include a mixer whichmultiplies a received signal by the frequency of a local oscillator, andlow-pass filters the incoming signal. The frequency of the receivedsignal is thus lowered to the intermediate frequency. In addition, theradio frequency parts 31 can comprise a part for adjusting the automaticgain control by which the signal level is kept substantially unchanged.The demodulator 32 converts the signal into digital in accordance withthe prior art, equalizes the signal, forms a symbol which corresponds tothe transmitted one and synchronizes the reception. The demodulatedsignal propagates further to the user. The demodulated locating signal,however, propagates to the locating signal measurement means 33 which,in accordance with the inventive method, by means of the signaldetermine the location of the terminal, the direction angle of thelocating signal and preferably even the intensity distribution. In thesolution in accordance with the block diagram of FIG. 3, the terminaldetermines its location itself in the locating means 34 by means of thebase station locations and the direction angles of the locating signals.From the locating means 34, the signal propagates typically to the userand/or elsewhere in the radio network. The locating means 34 can also besituated in the network part of the radio system.

The terminal shown in FIG. 4 performs the measurement of the locatingsignal, but the location determination of the terminal is performedelsewhere in the radio network. The terminal comprises an antenna 40, aduplex filter 41, radio frequency parts 42 of the receiver, ademodulator 43, radio frequency parts 44 of the transmitter, a modulator45 and means 46 for measuring the locating signal. In view of thereceiver, this solution operates in a highly similar way to the solutionin which the terminal determines its location itself. The antenna 40receives the locating signals. The duplex filter 42 separates thetransmitter and the receiver so as to enable them to operatesimultaneously. Consequently, the received locating signals alsopropagate from the antenna 40 through the duplex filter 41 to the radiofrequency parts 42, which convert the radio-frequency locating signalsto an intermediate frequency. The locating signals are demodulated inthe same manner as in the demodulator 32 of FIG. 3. The locating signalis measured in the measuring means 46, the reception moment of thesignal for instance being then determined. Furthermore, in the inventivesolution, the location and direction angles of the base stations can bedetermined in the measuring means 46, but the location and directionangles of the base stations can also be determined elsewhere in theradio system. Since the terminal does not determine its location itselfin this embodiment, the measurement information is transferred to themodulator 45 of the transmitter which converts the signal comprisingdigital symbols into analogue in a known modulation method (such as PAM,PSK, FSK, QAM, CPM, etc.) In addition, the modulator 45 can process thefrequency band of the signal thus formed. The radio frequency parts 44comprise a mixer by which the signal is multiplied into radio frequency.The radio-frequency measuring signal propagates through the duplexfilter 41 to the antenna 40, from which the locating signal furtherpropagates through the base station for instance to the base stationcontroller in order to form the location information of the terminal.

FIG. 5 shows a typical base station with its essential blocks inaccordance with the invention. The base station comprises a separatetransmitter 101 for the locating signal of the terminal. The transmitter101 comprises an antenna 56, radio frequency parts 57 and a modulator58. The antenna 56 can be directed mechanically or electronically in aknown manner. In the mechanical directioning, the antenna 56 rotatesphysically, whereas in the electronic directioning the phasing of theantenna elements included in the antenna 56 is changed in a known mannerso as to turn the direction of the beam. In the different embodiments ofthe invention, the information contents of the signal supplied to themodulator 58 can be empty or it can comprise information on the basestation and/or the direction of the transmission angle. The informationon the base station can be a code enabling the location of the basestation to be determined in a map or a table, or location information onthe base station.

The base station also comprises a receiver 201 and a transmitter 202 forthe actual operation of the radio system. More specifically, the basestation receiver comprises an antenna 50, duplex means 51 for separatingthe transmission and the reception directions, radio frequency means 52of the receiver and a demodulator 53. The transmitting side 202comprises a modulator 55 and radio frequency parts 54 from which thesignal propagates to the duplex means 51. The signal is furtherprocessed for a fixed network by a microprocessor.

Since the location of the terminal can be repeatedly determined atdifferent moments by the inventive solution, the solution of theinvention can also be used for determining the movement of the terminal.

Although the invention is described above with reference to the examplein accordance with the accompanying drawings, it is obvious that theinvention is not restricted thereto but it can be modified in many wayswithin the scope of the inventive idea disclosed in the attached claims.

What is claimed is:
 1. A locating method for determining the location ofa terminal in a radio system comprising a plurality of base stations andterminals, the method comprising: at least two base stations with knownlocations transmit a signal used in locating the terminal; the signalsare transmitted in beams with directional antenna patterns; thedirection of the beam is changed as a function of time; the terminalmeasures the signals transmitted by said at least two base stations,wherein on the basis of the terminal measurements, both a location ofthe at least two base stations and the transmission direction of thesignals used in location determination are determined; and the locationof the terminal is determined on the basis of the location of the basestations and the transmission direction of the signal used in locationdetermination.
 2. A method as claimed in claim 1, wherein the directionof the beam of the signals used in location determination is changed asa function of time by rotating the beam around the base station at thecell level.
 3. A method as claimed in claim 2, wherein the terminalmeasures the signal used in location determination during at least onefull 360° turn and, by means of the timing of the received signals, thesignal that has propagated via the most direct route from the basestation to the terminal is determined in order to reduce the effectscaused by multipath propagation.
 4. A method as claimed in claim 2,wherein the angular velocity ω of the rotation of the beam is steady, inwhich case the direction of the beam at an initiating rotational turn isωT at moment T, and at the beginning of at least one of the rotationalturns that follows the directioning of the beam is changed by phaseshift φ, the direction of the beam being ωT+φ at moment T.
 5. A methodas claimed in claim 1, wherein the terminal measures the intensitydistribution or the like of the beam during one turn in order todetermine the direction of the main axis of the beam, which main axis isused in determining the location of the terminal.
 6. A method as claimedin claim 1, wherein the terminal measures the intensity distribution orthe like of the beam in order to determine the direction of the mainaxis during several turns, which main axis is used in determining thelocation of the terminal.
 7. A method as claimed in claim 1, wherein thebase station transmits in time-divisional TDMA frames, when the beamrotates a full circle around the base station during one TDMA frame. 8.A method as claimed in claim 1, wherein the signals used in locationdetermination comprise information on the base station and on the angleinformation of the direction of the signal in order to determine thelocation of the terminal.
 9. A method as claimed in claim 1, wherein thesignals used in location determination comprise information on the basestation, the base stations and the terminals are synchronized so as tomake the reception moment and the transmission angle depend on eachother and the terminal measures the reception moment of the signal usedin location determination in order to determine the transmission angle.10. A method as claimed in claim 1, wherein when the network partcomprises the other parts of the radio system except the terminal andwhen the terminal transfers its measurement information to the networkpart, the location of the terminal is determined in the network part.11. A method as claimed in claim 10, wherein the location information istransferred from the network to the terminal over a radio path.
 12. Amethod as claimed in claim 1, wherein the terminal determines itslocation itself.
 13. A method as claimed in claim 1, wherein in a GSMradio system, the signal used in location determination is transmittedover a BCCH channel or the information on the frequency and time slotsof the signal used in location determination is provided over the BCCHchannel.
 14. A method as claimed in claim 1, wherein the signal used inlocation determination is transmitted at one frequency in one or moretime slots.
 15. A locating arrangement for determining the location of aterminal in a radio system comprising a plurality of base stations andterminals, the arrangement comprising: at least two base stations withknown locations comprise a transmitter for transmitting a signal used inlocating the terminal and having a directed beam, and the transmitter isarranged to change the direction of the beam as a function of time; theterminal comprises means in a receiver which are arranged to measure thesignals which are arranged to determine the location of the base stationand the transmission angle of the signal used in location determination,and the radio system comprises means for determining the location of theterminal on the basis of the measurement performed by the terminal. 16.An arrangement as claimed in claim 15, wherein the transmitter of thebase station is arranged to change the direction of the beam of thesignals as a function of time by rotating the beam around the basestation at the cell level.
 17. An arrangement as claimed in claim 15,wherein the terminal is arranged to measure the signal used in locationdetermination during at least one full 360° turn and the arrangement isarranged to determine, by means of the timing of the received signals,the signal that has propagated via the most direct route from the basestation to the terminal in order to reduce the effects caused bymultipath propagation.
 18. An arrangement as claimed in claim 15,wherein the transmitter of the base station is arranged to rotate theantenna beam steadily in regard to the angular velocity ω in which casethe direction of the beam at an initiating rotational turn is ωT, atmoment T, and at the beginning of at least one of the rotational turnsthat follows the transmitter is arranged to change the directioning ofthe beam by phase shift φ, the direction of the beam being ωT+φ at eachmoment T.
 19. An arrangement as claimed in claim 18, wherein theterminal is arranged to measure the intensity distribution or the likeof the beam during one turn in order to determine the direction of themain axis and to determine the location of the terminal.
 20. Anarrangement as claimed in claim 18, wherein the terminal is arranged tomeasure the intensity distribution or the like of the beam in order todetermine the direction of the main axis during several turns and todetermine the location of the terminal.
 21. An arrangement as claimed inclaim 18, wherein when the network part comprises the other parts of theradio system except the terminal and when the terminal transfers itsmeasurement information to the network part, the network part comprisesthe means to determine the location of the terminal.
 22. An arrangementas claimed in claim 21, wherein the network part is arranged to transferthe information on the location of the terminal over a radio path. 23.An arrangement as claimed in claim 15, wherein when the base stationtransmits in time-divisional TDMA frames, the transmitter of the basestation is arranged to rotate the beam a full circle around the basestation during one TDMA frame.
 24. An arrangement as claimed in claim15, wherein the signal used in location determination and transmitted bythe transmitter of the base station comprise information on the basestation and on the angle information of the direction of the signal inorder to determine the location of the terminal.
 25. An arrangement asclaimed in claim 15, wherein the signals transmitted by the transmitterof the base station and used in location determination compriseinformation on the base station, the base stations and the terminals arearranged to operate in a synchronized manner so as to make the receptionmoment and the transmission angle of the signal depend on each other ina known manner, and the terminal is arranged to measure the receptionmoment of the signal used in location determination.
 26. An arrangementas claimed in claim 15, wherein the terminal comprises the means todetermine its location itself.
 27. An arrangement as claimed in claim15, wherein in a GSM radio system, the transmitter of the base stationis arranged to transmit the signal used in location determination over aBCCH channel or provide the information on the frequency and time slotsof the signal used in location determination over the BCCH channel. 28.An arrangement as claimed in claim 15, wherein the transmitter of thebase station is arranged to transmit the signal used in locationdetermination at one frequency in one or more time slots.